From ec5ce0aeaedbccd9865c7fb2503e6c91488d3054 Mon Sep 17 00:00:00 2001
From: David Scandurra <david.scandurra@check24.de>
Date: Sat, 30 Aug 2025 15:05:08 +0200
Subject: [PATCH] Fix setting known offset

---
 src/Analyser/MutatingScope.php                  |  7 +++++++
 .../data/offset-access-value-assignment.php     | 17 +++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index 57d047fbf8..3a2d364900 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -750,6 +750,13 @@ public function getType(Expr $node): Type
 			return $this->getType($node->getVar())->unsetOffset($this->getType($node->getDim()));
 		}
 		if ($node instanceof SetOffsetValueTypeExpr) {
+			$var = $node->getVar() instanceof OriginalPropertyTypeExpr ? $node->getVar()->getPropertyFetch() : $node->getVar();
+			if ($node->getDim() !== null && $this->hasExpressionType(new Expr\ArrayDimFetch($var, $node->getDim()))->yes()) {
+				return $this->getType($node->getVar())->setExistingOffsetValueType(
+					$this->getType($node->getDim()),
+					$this->getType($node->getValue()),
+				);
+			}
 			return $this->getType($node->getVar())->setOffsetValueType(
 				$node->getDim() !== null ? $this->getType($node->getDim()) : null,
 				$this->getType($node->getValue()),
diff --git a/tests/PHPStan/Rules/Arrays/data/offset-access-value-assignment.php b/tests/PHPStan/Rules/Arrays/data/offset-access-value-assignment.php
index 2e4f00d7e3..bac11303d3 100644
--- a/tests/PHPStan/Rules/Arrays/data/offset-access-value-assignment.php
+++ b/tests/PHPStan/Rules/Arrays/data/offset-access-value-assignment.php
@@ -59,3 +59,20 @@ public function foo(): void
 		$this->collection2[] = 2;
 	}
 }
+
+class TestList
+{
+
+	/**
+	 * @var list<int>
+	 */
+	public array $list = [];
+
+	public function setKnownOffset(int $offset): void
+	{
+		if (isset($this->list[$offset])) {
+			$this->list[$offset] = 123;
+		}
+	}
+
+}